This invention relates to a process for solidifying high-level radioactive wastes for storage. More specifically, this invention relates to an improvement in the process for preparing glasses for storage from calcined high-level radioactive wastes and glass frit.
The chemical reprocessing of spent nuclear reactor fuel elements to recover the unburned nuclear reactor fuel material generates aqueous solutions containing radioactive wastes. The aqueous waste solutions contain nitric acid and present difficult problems in their handling and storage. Since it is necessary to store these radioactive wastes for extremely long periods of time to permit decay of the highly radioactive fission products included in the wastes, the aqueous wastes are converted to solid form which, in addition to occupying less volume than the corresponding liquid wastes, is less corrosive and more durable for handling and long-term storage. These aqueous radioactive waste solutions can be converted to a solid form by several processes such as spray solidification, fluidized-bed calcination and pot calcination.
In order to protect the environment and prevent the escape of highly radioactive material from the long-term storage facilities, it is necessary that the stored solid be highly impervious to attack by outside influences which may be present in the storage facility, such as water. Since most calcines contain leachable material, it becomes necessary that they either be stored in suitable containers or converted into a chemically inert substance. However, since container materials are subject to corrosion and may ultimately leak and allow the escape of radioactive material into the environment, it is preferred that the material placed in the container also be chemically inert. For this reason, considerable effort is being made to develop economical and effective processes for converting the highly radioactive calcines into glass. Conversion of calcines into glass is attractive because glass is relatively inexpensive, inert and generally resistant to leaching. Glass is easy to prepare to mixing the calcine with a glass frit, which is a mixture of silica and fluxes such as boron oxide and sodium oxide which, when melted with the calcine, will form glass, melting the resulting mixture to form the molten glass and pouring the molten glass into a metal storage container to form a glass casting containing the highly radioactive solid waste materials. It has been found that glasses prepared from calcines formed from high-level radioactive wastes from power reactors, separated into two phases when solidified rather than forming a single liquid phase -- the second upper phase being highly leachable, thus rendering the glass unacceptable for the storage of radioactive wastes. Tests showed that this second phase is (Na,K) MoO.sub.4 containing some strontium and cesium and resulted from reaction between molybdenum fission products generally present as molybdenum (VI) compounds, the calcine and alkali metals found in both the calcine and the glass frit.
A number of methods have been tried to remedy the problem, but none has been completely successful. For example, phosphate glasses have been tried; however, they are more readily subject to devitrification whereupon they become highly leachable. Attempts have been made to change the composition by decreasing the sodium content or increasing the boron oxide content -- neither of which produced completely satisfactory results. Another solution is to dilute the calcine more by increasing the amount of glass present -- but this increases the cost of the glass and greatly increases the amount of waste for which expensive storage must be provided.